1. Field of the Invention
The present invention relates to a control circuit for controlling the operation of an impact drive actuator in which a piezoelectric element is used; to an image stabilization control circuit for compensating for shaking of an imaging device using the control circuit; and to an imaging device that uses the image stabilization control circuit.
2. Description of the Related Art
Piezoelectric actuators that utilize the electrostriction effects of a piezoelectric element have conventionally been used for camera shake compensation, autofocus, and the like, for example.
An impact drive piezoelectric actuator has a drive shaft for retaining a movement object with the aid of friction, and the drive shaft is extended and retracted by a piezoelectric element. A drive circuit generates a voltage signal whereby the drive shaft has a different speed during extension and during retraction, i.e., a voltage signal whereby the drive shaft slowly extends and rapidly retracts, or does the opposite, and the drive circuit applies the voltage signal to the piezoelectric element. During rapid movement of the drive shaft, the movement object slides with respect to the drive shaft due to inertia and remains substantially in position. If the drive shaft is moved slowly, the position of the movement object is displaced along with the drive shaft by static friction. The movement object can by displaced in a specific direction with respect to the drive shaft by this extension and retraction.
This displacement can be achieved in principle by applying a voltage signal that has a jagged waveform having different rising times and falling times to the piezoelectric element. However, the drive signal outputted from the drive circuit need not necessarily have a jagged waveform, and it is known that the displacement described above can be obtained by adjusting the frequency or duty ratio even when the signal has a square waveform. The movement speed of the movement object is dependent on the frequency and duty ratio of the drive signal, and the moving direction can be varied by inverting the duty ratio, for example.
In servo control using an actuator, it is possible to follow to the target position with good precision by acquiring the newest position each time the actuator is driven and the object is displaced. However, the amount of displacement in one cycle of a piezoelectric actuator driven in accordance with a drive pulse is generally minute, and when a large amount of displacement is required, problems occur in that the target position is not approached closely enough by a single cycle of servo control.
The follow speed can be increased by reducing the length of the servo control cycle. However, the shortening of the servo control cycle is sometimes limited. In an image stabilization system in particular, the position of the lens or imaging element must be controlled in at least two dimensions, and a plurality of position sensors and angular speed sensors is used to detect the position and oscillation of the lens or the like. In order to reduce the size of the control circuit, an A/D converter is shared by time division to acquire the outputs of the plurality of sensors. In such a case, reduction of the servo control cycle length can be limited by the time division operation.